1. Field of the Invention
The present invention concerns in general terms the protection of electrical systems against load dump. It applies in particular in the automobile field.
2. Description of the Related Art
In a motor vehicle, an electrical system, referred to as an “on-board system”, serves to supply the electrical equipment with which the vehicle is equipped. Such an on-board system can be assimilated to a DC supply bus. The supply is provided by at least one battery. The latter is recharged by virtue of a rotary electrical machine, from the energy supplied by the rotation of the thermal engine of the vehicle. The rotary electrical machine means more generally any single- or polyphase rotary electrical machine for producing output DC current supplying the on-board system. It may in particular be an alternator or an alternator/starter.
In the event of abrupt disconnection of an electrical load in the on-board system, or of a battery, or both, a phenomenon of load dump (“load-dump” in English) is created, which causes an overvoltage on the on-board system. This is because, since a regulation of an inducing current in the machine cannot act sufficiently quickly following the load dump, the machine continues to deliver the same output current whereas the current consumption on the on-board system side has dropped.
Conventionally, the vehicle battery is a 14 volt battery. In principle, by virtue of its low internal resistance, it limits to approximately 17 volts the voltage peaks that occur on the on-board system of the vehicle in the event of load dump. The battery thus absorbs the small overvoltages. Nevertheless, in the event of disconnection of the battery (due to the breakage of a supply cable, for example), a very high overvoltage may occur on the on-board system. This is because the current delivered by the machine charges the capacitances (including the stray capacitances) connected to the on-board system, and consequently causes the DC voltage of the on-board system to increase significantly.
This overvoltage risks damaging the electrical equipment supplied by the on-board system. This is why all the electrical equipment on the vehicle is sized to withstand a maximum voltage of approximately 32 volts, which corresponds to an overvoltage of approximately 20 volts.
Various solutions are known for limiting the voltage on the on-board system to a maximum acceptable voltage, that is to say the highest voltage that the electrical equipment on the vehicle can withstand without risk of damage.
For example, the document WO 03/032465 proposes a first embodiment using, for switches in a power circuit, power MOSFET transistors that are calibrated for voltage in order to work by avalanche effect for a given voltage, lower than the maximum acceptable voltage. Thus, in the event of load dump, the limitation of the voltage of the on-board system is guaranteed by the power transistors constituting the switches of the bridge rectifier going into avalanche.
The same document WO 03/032465 also mentions a conventional solution consisting of adding Zener diodes to the on-board system in order to limit the voltage of the on-board system.
Other solutions consist of adding an additional load in the on-board system in order to absorb the surplus energy in the event of load dump.
These known solutions have certain advantages but are imperfect in the context of the future use of batteries of larger capacity, for example 42 volts, in vehicles, made necessary by the tendency towards the increase in electrical equipment with high energy consumption. This is because, according to the specifications of a future European Standard (still in the course of drafting), the electrical equipment of the on-board system of the vehicle will probably have to be sized to function up to a voltage of 48 volts and to withstand a maximum voltage of 58 volts, which corresponds to an overvoltage of only 10 volts.
However, the limitation voltage or clipping voltage of MOSFET transistors is too high (above 58V) and is not sufficiently controlled to enable them to be used in this context. This is because the clipping value of MOSFETs depends in particular on a parameter such as temperature.
In addition, in the case of the use of Zener diodes with MOS transistors, they must be able to absorb currents of several hundreds of amperes, giving rise to significant bulk and additional cost since it is necessary to add several of them. The same applies for the additional loads.
What is needed, therefore, is an improved system and method for protecting an electrical system that also overcomes one or more of the problems in the prior art.